Run Multiple Debian Versions Simultaneously

Happily, at the moment it's still early on in the Debian release cycle, so not too many packages are missing from, or outdated in, stable. However, it can be really useful to grab packages from upstream when needed without necessarily upgrading your whole system to testing. This is quite straightforward to do with apt, but it isn't all that well-documented.

Tip of the Trade: Meet apt -- the best-kept secret for keeping up with the Debian release cycle.

apt-get prefers options with higher pin values, so this prioritizes
stable over testing.

To install a single package from testing, use apt-get install packagename/testing. However, this doesn't grab any
dependencies, so you may not be able to run the upgrade. The alternative is
to use apt-get install -t testing packagename, which will try to meet
the dependencies if possible.

Note, however, that sometimes it's not. For example, if the upgrade
will break other packages, you may just have to make do without it!